Pressure senser



Dec. 20, 1960 P. J. LEESON ETAL PRESSURE SENSER 2 Sheets-Sheet 2 FiledNOV. 20, 1957 q WWW w T mm a o ew mdm/ c I United States Patent PRESSURESENSER Plato J. Leeson and William F. Weatherford, Rockford,

Ill., assignors to Woodward Governor Company, Rockford, Ill., acorporation of Illinois Filed Nov. 20, 1957, Ser. No. 697,626

6 Claims. (Cl. 137-785) This invention relates to a device for measuringpressure changes in a fluid and including a yieldable wall, such as adiaphragm or a bellows exposed on one side to the pressure fluid so thatthe bellows expands and contracts in response to pressure changes.Devices of this type may be used in gas turbine controls, such as in anaircraft engine, to measure the pressure changes in the air admitted toor discharged from the compressor in which case the controls includingthe pressure senser are enclosed in a casing filled with liquid underpressure. In order that changes in pressure of the surrounding mediumsuch as this liquid do not effect the action of the bellows, the latteris opposed by a reference bellows which is evacuated so that thepressure being measured is opposed by a constant pressure.

The primary object of the invention is to provide a pressure senser ofthe above character which continues to perform its intended pressuresensing function in the event of a failure of one of the bellows.

A more detailed object is to employ a third bellows which is subjectedto a second constant pressure and which normally is inactive but whichbecomes active to oppose the main bellows in the event that thereference bellows fails.

The invention also resides in the novel arrangement by which thepressure senser is able to continue to function even in the event of afailure in any one of the three bellows.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in Which Figure 1 is a fragmentary sectional viewof a pressure senser embodying the novel features of the presentinvention.

F Fig. 2 is a sectional view taken along the line 22 in F Fig. 3 is asectional view taken along the line 3-3 in Fig. 4 is an enlargedfragmentary view of the novel connection which renders the auxiliarybellows active when the reference bellows fails.

Figs. 5, 6 and 7 are schematic views respectively illustrating theaction of the pressure senser when the main, reference and auxiliarybellows fail.

As shown in the drawings for purposes of illustration, the pressuresenser 10 of the present invention is adapted for use in conjunctionwith a gas turbine of the type which may be employed in an aircraftengine. Fuel is supplied to the engine under the control of a suitablevalve (not shown) which is activated by a rod 11. The latter moves inresponse changes in a pressure P! which herein is the pressure at theinlet of the compressor of the engine, and this pressure is measured bythe pressure senser 10. For this purpose, the pressure senser includes aprimary yieldable wall, herein a bellows 12, which is disposed within acasing 13 with its outer end anchored to an inturned flange at the upperend of a 2,965,137 Patented Dec. 20, 1960 tube 13 telescoped into thecasing 13 and held therein by screws 13. The interior of the bellowscommunicates with the compressor inlet through a restricted passage 14.The casing 13 is filled with a liquid which in this case is the fuel forthe engine and is under a pressure P0.

In order to convert changes in the pressure Pt into movement of the fuelvalve activating rod 11, a yoke 15 is secured to and depends from a wallmember in the form of a plate 16 secured to and closing the freelymovable inner end of the bellows 12. An abutment 17 on a stem 18 whichdepends from the lower end of the yoke engages one end of a lever 19fulcrumed at 20. The other end of the lever seats against the lower endof a compression spring 21 whose other end abuts against the undersideof the piston 22 of a servo 23. The servo piston slides in a cylinder 24and is connected to the valve rod 11. On the lower end of the stem 18 isa valve 25 which, through a port 26 and a line 27, connects the upperend of the cylinder 24 either with a pressure source P through a port 28or to drain through a port 29.

With the foregoing arrangement, the parts are in the position shown inFig. 1 when the system is in balance, that is, the force of the spring21 balances the pressure in the cylinder 24 above the piston 22 and thespring force also, acting through the lever 19 and the yoke 15, balancesthe pressure P1 plus the spring force of the bellows 12 and the force ofa calibrating spring 30. The latter acts in compression between a seat30 on the casing 13 and a head 30' on the upper end of a stem 31projecting up into the bellows from the plate 16.

Upon an increase in the pressure Pt the bellows 12 expands moving thestem 18 down. This turns the lever 19 counterclockwise as viewed in Fig.1 so that the spring 21 is compressed and urges the piston 22 upwardly.Simultaneously, the pilot valve 25 connects the ports 28 and 26 with aresulting build up of pressure in the cylinder 24. The increase ofpressure continues until the piston moves down sufiiciently to returnthe lever 19 and the pilot valve to their original positions andrestores equilibrium to the system. This movement of the piston shiftsthe rod 11 and changes the setting of the fuel valve an amountcorrelated with the change in the pressure P13 With a decrease of thepressure Pt the pilot valve 25 connects the cylinder 24 to drain and thepressure in the cylinder decreases. This results in the piston 22 movingup until the reduced pressure balances the force of the spring 21 andthe lever 19 returns to the normal position. Thus, the setting of thefuel valve is changed in the opposite direction.

Since the casing 13 is filled with fuel under pressure P0 and since thispressure varies, the bellows 12 is ar ranged to act in opposition to asecond yieldable wall such as an intermediate reference bellows 32 whichis submerged in the fuel and is evacuated to O p.s.i.a. so that thesystem responds to changes in the absolute pressure at the compressorinlet. The free and movable end of the reference bellows is secured tothe closed inner end of the bellows 12 by the plate 16 through themedium of a threaded connection 33 between the stem 31 and a stem 34projecting into the reference bellows. The latter is fastened at itsother end to a stationary part in the form of a ring 35 made r'gid withthe casing 13 through the medium of arms 35 (Fig. 2) projecting fromopposite sides of the ring and suitably fastened to the casing 13. Theinterior of the bellows 32 is evacuated through a tube 36 which projectsthrough the stem 34 and is pinched closed after the evacuation.

According to the present invention, the pressure senser 10 isconstructed and arranged in a novel manner so that it continues toperform its intended function even in the event of a rupture or failureof one of the bellows. To

this end, the pressure senser employs a third or auxiliary yieldablewall, herein a bellows 37, which is closed by a wall 37 at its outer endand normally is inactive but becomes active upon failure ofthe referencebellows. llhe auxiliary bellows'is subjected continuously to a'constantpressure slightly above the zero pressure in the reference bellows andnormally is not connected tothe. main bellows 12. Through a lost motionconnection 38, however, the auxiliary bellows 37 opposes the mainbellows 12 upon failure of the reference bellows 32.

In the present instance, one end of the auxiliary bellows 37 is anchoredto the ring 35 in axial alinement with the first and intermediatebellows 12 and 32-and this bellows is enclosed within a cup-shapedhousing 39. The interior of the latter is evacuated to a pressurePrwhich herein may be on the order of 5 p.s.i.a. and constitutes anauxiliary reference pressure. The interior of the auxiliary bellows 37communicates with the interior. of the reference bellows 32 through apassage 35' atthe center of the ring 35 and thus both bellowsareevacuated' to 0 p.s.i.a.

To form the lost motion connection 38, opposed abutments 4i and 41 arecarried by the free endsof the reference bellows 32 and the auxiliarybellows 37 respectively. These abutments are spaced apart a distancegreat enough that they do not engage each other in the normal operationof the pressure senser 10 but do engage under certain conditions offailure as will be explained in detail subsequently. Herein, theabutment 41 is an en larged head on the upper end of rod 42 upstandingfrom the lower or free end of the auxiliary bellows 37. The head 41 isreceived in a bore 43'forrned in the lower. end of the stem 34 andshouldered at its lower end to constitute the abutment 40.

When the pressure senser 10 is used in connection with the gas turbineof an aircraft engine, the casing 13 may conveniently be filled withfuel under a pressure (Po) which normally ranges between 20 and 25p.s.i.a. The compressor inlet pressure (P1 varies with the speed andaltitude of the aircraft and may rangefrom 0.5 to 40 p.s.i.a. althoughusually it is less than the pressure P0. As stated earlier, theinteriors of the bellows 32 and 37 are evacuated to 0 p.s.i.a. while theinterior of the housing 39 is evacuated to a pressure (Pr) on the orderof 5 p.s.i.a. Under these conditions, the pressure Pt within the bellows12 opposes the zero pressure inside the bellows 32. Thus, the forceapplied to the left end of the lever 19 is a combination of the forcedue to the pressure Pt and the forces of the spring efiects of thebellows 12 and 32 and the spring 30, the bellows 37 being completelyinactive. As a result, movement of the yoke 15, and hence of the valverod 11, is a direct measure of the change in the pressure P2 During suchmovement, the abutments 4t) and 41 remain out of engagement.

Should the main bellows 12 fail as illustrated by the arrow in Fig. 5,the fuel under the pressure P0 in the casing 13 will leak into thisbellows providing the pressure P0 is greater than the pressure Pi as itnormally is. This results in an increased flow of fuel to the gasturbine for two reasons. First, the fuel leaking into the bellows 12flows to the compressor inlet, although this flow is comparatively smalldue to the restricted passage 14. Second, the pressure inside thebellows 12 increases slightly shifting the yoke 15 down as shown in-Fig.5. As explained previously, this causes the valve rod 11 to move in thevalve opening direction. Although there is an increase in the supply offuel, this increase is comparatively small and is w'thin the limits ofsafe operation of the aircraft engine. Moreover, the bellows 12continues to respond to changes in the pressure at the compressor inletso that this bellows still performs its intended function.

If the case pressure P0 should be less than the pressure BI the pressureinside the main bellows 12 will leak out. This will cause a slightreduction in the latter pressure and hence a small reduction in the fuelvalve setting. This reduction is within the limits of safe operationand, as in the previous case, the bellows 12 continues to expand andcontract with changes in the pressure at the compressor inlet.

A failure of the reference bellows 32 also causes only a slight changein the setting of the fuel valve and permits continued safe OPrationof'the pressure senser 10. With such a failure as indicated by the arrowin Fig. 6 the case pressure P0 enters the interiors-of the'bellows- 32and 37. As aresultythesfree end'of the "reference bellows 32 moves upwhile the free end of'the auxiliary bellows 37 moves down thus taking upthe available slack in the lost motion connection and bringing theabutments 40 and 41 into engagement as illustrated in Fig. 6. In thisway, the pressure within these two bellows in effect locks the free endstogether so that the pressure Pt now opposes the pressure Pr in thehousing 39. Sincethelatter pressureis slightly higher than the 'zeropressure which originally oppose the pressure Pt the yoke 15 is moved,up. and the fuel valve is closed a small. amount. Once the casepressure has filled the bellows 32 and 37, the main bellows 12 continuesto respond to the pressure Pt and controls the fuel supply-tov the gasturbine. The decrease of the fuel under these conditions has been foundto be 13 percent or less, this being within the safe limits.

5 The pressure senser. 1 0, also continues to respond to the pressure atthe compressor inlet and to control the fuel valve when the auxiliarybellows 37 fails as indicated-by the arrow in Fig. 7. In'that case, thepressure Pr leaks into the bellows 32 and 37 and causes engagement ofthe abutments 40 and 41. The pressure within these two bellows andwithin the housing then isthe same and is less than the originalpressure Pr and greater than the zero pressure originally withinthebellows. Normally, this would raise the yoke 15 and produce a decreasein the fuel supply rate. At this time, however, there is no pressurecounterbalancing the force due to the spring effect of the auxiliarybellows37r In the illustrated embodiment, this force is greater than theeffect of the increased pressure within the two bellows 32and 37 so thatthe, net result is a downward movement. of the yoke and an increase inthe fuel supply rate,.the .increase being on the order of 10 percent orless. Although the pressure P1 now is opposed by a slightly differentpressure, the valve rod 11 still moves in response. to changes .in thepressure P2 It will he observed that a pressure senser constructed inaccordance with the invention continues to respond to changes in thepressure Pt even if. any one. of the bellows 12, 32 and 37 should fail.Although the position of the rod 11 and hence of the fuel valve. ischanged somewhat in the event of such a failure, thedeviation from thedesired setting is small and falls well withirrthe limits of safeoperation when the pressure senser busted in connection with thecontrols of anaircraft-engine. In other words, a failure of one of thebellows simply causes a minor shifting of the fuel scheduling. for theengine while the pressure senser continues its functions of measuringthe pressure at the compressor inlet and controlling the fuel schedulein response to changes of this pressure. It should be observed that: thep ressure Pr may be 0 p.s.i.a. but it is preferred to use a pressuresomewhathigher so that a failure of either. of the bellows 32 and 37will cause a change in the fuel scheduleand indicate a failure whichshould be corrected.

We claim as our invention:

1. A pressure sensing device comprising'a. casing, a liquid underpressure within said casing, a first bellows disposed within said casingand having one end anchored to the casing, a second bellows disposedwithin said casing and alined with said first bellows, a wall memberclosing the free end of said first bellows and-connecting the same. tothe adjacent end of said second bellows, a

stationary part disposed adjacent and anchored to the other end of saidsecond bellows, means for admitting a fluid under pressure to theinterior of said first bellows, said second bellows being evacuatedwhereby said member moves in response to changes in pressure of saidfluid, a third bellows disposed within said casing in alinement withsaid second bellows and having one end anchored to said stationary part,the other end being closed and freely movable, a passage connecting theinteriors of said second and third bellows whereby both bellows areevacuated, a housing enclosing said third bellows and being evacuated toa pressure greater than the pressure within said third bellows, and alost motion connection within said second and third bellows comprisingelements rigid with the free ends of said second and third bellowsrespectively and having opposed abutments thereon spaced apart to engageand limit relative movement of the free ends of said second and thirdbellows upon abnormal movement of said wall member.

2. A pressure sensing device comprising a casing, a first bellowsdisposed within said casing and having one end anchored to the casing,means for admitting a fluid under pressure to the interior of saidbellows, a second bellows disposed within said casing with one endanchored to the casing and with the free end thereof opposing the freeend of said first bellows, a wall member connecting and closing the freeends of said two bellows, said second bellows being evacuated wherebysaid member moves in response to changes in the pressure of said fluid,a third bellows disposed within said casing and having one end anchoredto said casing, the other end being free, and closed, a passageconnecting the interiors of said second and third bellows whereby bothare evacuated, a housing enclosing said third bellows and beingevacuated to a pressure greater than the pressure within the thirdbellows, a fluid under pressure filling said casing and surrounding saidfirst and second bellows, a first element connected to the free end ofsaid second bellows and extending axially along the interior of thelatter, a second element connected to the free end of said third bellowsand extending axially along the interior of the third bellows, at leastone of said elements projecting through said passage to overlap theother element, and opposed surfaces on said elements normally spacedapart and operable to limit relative expansion of said second and thirdbellows to a degree in excess of the normal movement of said member.

3. A pressure sensing device comprising a casing, a liquid underpressure within said casing, a first bellows disposed within said casingand having one end anchored to the casing, a second bellows disposedwithin said casing and alined with said first bellows, a wall memberclosing the free end of said first bellows and connecting the same tothe adjacent end of said second bellows, a stationary part disposedadjacent and anchored to the other end of said second bellows, means foradmitting a fluid under pressure to the interior of said first bellows,said second bellows being evacuated whereby said member moves inresponse to changes in pressure of said fluid, a third bellows disposedwithin said casing in alinement with said second bellows and having oneend anchored to said part, the other end of said third bellows beingclosed and freely movable, a passage connecting the interiors of saidsecond and third bellows whereby both bellows are evacuated, a housingenclosing said third bellows and being evacuated to a pressure greaterthan the pressure within said third bellows, and opposed abutmentsrigidly connected respectively to the free ends of said second and thirdbellows and spaced apart to engage and limit relative movement of thosefree ends upon abnormal movement of said member.

4. A pressure sensing device comprising a casing, a first bellowsdisposed within said casing and having one end anchored to the casing,means for admitting a fluid under pressure to the interior of saidbellows, a second bellows disposed within said casing with one endanchored to the casing and with the free end thereof opposing the freeend of said first bellows, a wall member closing the free ends of saidtwo bellows and joining these ends together, said second bellows beingevacuated whereby said member moves in response to changes in thepressure of said fluid, a third bellows disposed within said casing andhaving one end anchored to the casing, the other end of the bellowsbeing closed and freely movable, a passage connecting the interiors ofsaid second and third bellows whereby both are evacuated, a housingenclosing said third bellows and being evacuated to a pressure greaterthan the pressure within the third bellows, a fluid under pressurefilling said casing and surrounding said first and second bellows, and alost motion connection between the free ends of said second and thirdbellows and operable to limit relative expansion of the two bellows to adegree in excess of the normal movement of said member.

5. A pressure sensing device comprising a casing, a liquid underpressure within said casing, a first bellows disposed within said casingand having one end anchored to the casing, a second bellows disposedwithin said casing and alined with said first bellows, a Wall memberclosing the free end of said first bellows and connecting the same tothe adjacent end of said second bellows, a stationary part disposedadjacent and anchored to the other end of said second bellows, means foradmitting a fluid under pressure to the interior of said first bellows,said second bellows being evacuated whereby said part moves in responseto changes in the pressure of said fluid, a third bellows disposedwithin said casing in alinement with said second bellows and having oneend anchored to said part, a wall closing the other and free end of saidthird bellows, a passage connecting the interiors of said second andthird bellows so that both bellows are evacuated, a housing enclosingsaid third bellows and evacuated to a pressure greater than the pressure within the third bellows, and a lost motion connection projectingthrough said passage and comprising elements respectively rigid withsaid wall and wall member, said elements having opposed abutmentsthereon spaced apart to engage each other and limit separation of thefree ends of said second and third bellows upon abnormal movement ofsaid wall member.

6. A device for sensing changes in a control pressure comprising first,intermediate, and third bellows arranged in end to end relation along acommon axis with the inner ends of the first and third bellowsrespectively joined to opposite ends of the intermediate bellows, a wallmember closing the connected ends of said first and intermediate bellowsand mounted for movement along said axis, a stationary part supportingthe connected ends of said intermediate and third bellows, a wallclosing the outer end of said third bellows and normally movablerelative to said wall member along said axis through a limited normaloperating range, said intermediate and third bellows communicating witheach other and being evacuated, a stationary housing enclosing saidthird bellows and defining a space around the latter evacuated to apressure slightly higher than the pressure within the third bellows, afluid filled casing supporting said stationary part and enclosing saidfirst and intermediate bellows and said housing, and means operable toprevent separation of said wall and wall member beyond said range.

References Cited in the file of this patent UNITED STATES PATENTS2,778,350 Reggio Jan. 22, 1957

